Ring laser gyroscopes (RLG) include reflectors (typically 3 or 4 reflectors) aligned to direct counter propagating optical beams in a lasing plane that is perpendicular to the sense axis of the ring laser gyroscope. A gyroscope frame holds the ring laser gyroscope in a plane referred to herein as a reference mounting plane.
The ring laser gyroscope includes a gyro mounting feature that is screwed onto the gyro frame. Any unintentional contact between the gyro mounting feature and a screw that attaches the gyro mounting feature to the gyro frame can cause a shift in the critical angular relationship between the lasing plane and the reference mounting plane. This shift is exacerbated over temperature swings to which gyros are typically exposed.
FIGS. 8A and 8B show a prior art screw 650 inserted through a gyro mounting feature 31 attached to a gyro frame 100 at a first temperature T1 and a second temperature T2, respectively. The entire length L of the shaft of the prior art screw 650 is threaded. The prior art screw 650 is situated in a through hole 125 formed in the gyro mounting feature 31. A distal end of the screw 650 is screwed into a screw hole 110 in the gyro frame 100. As shown in FIG. 8A, the lasing plane is spanned by axes (XL, YL) and the reference mounting plane is spanned by the axes (XM, YM). The axis ZL is perpendicular to the lasing plane (XL, YL) and the axis ZM is perpendicular to the reference mounting plane. In FIG. 8A, the critical angular relationship between the lasing plane (XL, YL) and the reference mounting plane (XM, YM) is indicated by the angular offset αx between the XM and XL axes and by the angular offset αz between the ZM and ZL axes. The gyroscope is calibrated to account for this small angular offset between the lasing plane (XL, YL) and the reference mounting plane (XM, YM).
As shown in FIG. 8A, one thread 655 unintentionally contacts the side wall 119 of the through hole 125 when the device is at first temperature T1. As shown in FIG. 8B, the thread 655 bites further into the side wall 119 of the through hole 125 as the temperature shifts from a first temperature T1 to a second temperature T2. This change in temperature thus causes a slight shift in the small angular relationship (ax, az) between the lasing plane (XL, YL) and the reference mounting plane (XM, YM) shown in FIG. 8A. In FIG. 8B, the critical angular relationship between the lasing plane (XL, YL) and the reference mounting plane (XM, YM) is indicated by the angle βx between the XM and XL axes and by the angle βz between the ZM and ZL axes. Thus, the critical angular relationship between the lasing plane and the reference mounting plane wobbles between at least two different angles.
When the lasing plane (XL, YL) shifts with reference to the reference mounting plane (XM, YM), the original calibration of the ring laser gyro is no longer accurate. If the angular relationship between the lasing plane (XL, YL) and the reference mounting plane (XM, YM) is wobbling due to temperature effects, any attempts to recalibrate only provide a temporary fix. Less than ideal information (inaccurate data) from the gyro degrades the navigation performance.
For a multi-axis ring laser gyroscope, in which two or more ring laser gyroscopes (each having a gyro mounting feature) are orthogonally mounted with respect to each other, an unintentional contact between one or more of the two or more gyro mounting features and the screw attaching the gyro mounting feature to the gyro frame can cause a shift in the critical angular relationship between the sense axes of each of the two or more ring laser gyroscopes.